Ken Ishimaru¹, Naoki Hirotsu¹, Yuka Madoka¹, Takayuki Kashiwagi¹, Kazuhiro Ujiie¹, Etsuko Katoh²

¹Functional Plant Research Unit, ²Biomolecular Research Unit

［Abstract］

We identified a novel gene for grain length and weight, THOUSAND-GRAIN WEIGHT 6 (TGW6) from the indica rice cultivar Kasalath. TGW6 encodes a novel protein related to indole-3-acetic acid (IAA) synthesis. Loss of function of the Kasalath allele enhances grain length and weight with pleiotropic effect on sink and source associated with IAA synthesis.

［Keywords］

grain length, grain weight, sink, source, yield

［Background］

Grain length is one of the main determinant factors of rice yield. A substantial expansion in sink size has been achieved in most Japanese rice cultivars and the source ability in determining the supply of carbohydrate to the sink has been identified as a limiting factor in yield. A novel gene TGW6 that increases grain length and weight with pleiotropic effects on sink and source has been identified in the indica rice cultivar Kasalath.

［Results and Discussion］

1. We characterized the TGW6 alleles in both the indica cultivar Kasalath and the japonica cultivar Nipponbare. The Nipponbare TGW6 encodes a novel protein that hydrolyzed indole-3-acetic acid (IAA)-glucose into IAA and glucose. The corresponding allele in Kasalath consists of a 1-bp deletion which causes loss of function thereby enhancing grain length and weight.
   
2. During endosperm development, cell division is halted and multiplication of the primary endosperm nucleus produces a multinucleate cell. The length of this syncytial period determines the cell number and consequently the grain length. Nipponbare TGW6 seems to affect the timing of the transition from this syncytial period to the cellular phase by controlling the supply of IAA and thereby shortening the syncytial phase and then which consequently limits the cell number and grain length (sink size) comparing to Kasalath TGW6.
   
3. The carbohydrate which accumulates in a plant body before heading is supplied to the developing endosperm after heading. The Nipponbare TGW6 induced the downregulation of genes related to starch synthesis resulting in reduced accumulation of carbohydrate supplied to the developing endosperm (source ability). When the leaf sheaths, the main organ that accumulates carbohydrate were treated with IAA before heading, the expression of genes related to starch synthesis was downregulated. Therefore, in addition to the effect on sink size, the effect of the Nipponbare allele on source ability maybe associated with IAA synthesis. Loss of function in the Kasalath allele had desirable effects on grain length and weight (Fig. 1).
   
4. The analysis of genetic diversity shows that only a few cultivars have the Kasalath-type TGW6. This allele has not been identified in high yielding cultivars such as Takanari and IR64 as well as the Japanese modern cultivars including Koshihikari. In a near isogenic line with Koshihikari genetic background (NILTGW633), Kasalath TGW6 had a positive effect on grain length and weight (Fig. 2). High temperatures at the ripening stage normally induce a decrease in rice grain yield and quality in Koshihikari. The damage was reduced in NILTGW633 with maintenance of grain number in comparison to the Koshihikari (control).

1. The Kasalath-type TGW6 allele can be used in improving the yield of most Japanese rice cultivars. We have developed a new Koshihikari near isogenic line by narrowing the introduced chromosome segment of Kasalath for more efficient utilization as a new breeding line.

Fig.1. Schematic model showing the effect of Nipponbare and Kasalath TGW6 on grain length and weight mediated by IAA.

Fig.2. Grain length and weight in Koshihikari (control) and the Koshihikari near isogenic line with Kasalath TGW6 (NILTGW633). ***, P < 0.0001.

 

[Collaborator]
Bunichi Shimizu (Toyo Univ.), Tsuyoshi Miyakawa (Kyoto Univ.)

[References]

1. Ishimaru K, Hirotsu N, Madoka Y, Murakami N, Hara N, Onodera H, Kashiwagi T, Ujiie K, Shimizu B, Onishi A, Miyagawa H, Katoh E (2013) Loss of function of the IAA-glucose hydrolase gene TGW6 enhances rice grain weight and increases yield. Nature Genetics 45(6):707-711
   
2. Patent #5288608 (Japan)